The present invention relates to a process for the regeneration of ion exchange beds used in the reclamation of alkanolamine solutions. In particular, the present invention relates to a process for removing anions that build up on the resins in the ion exchange beds by purging the beds with reagents that remove the anions.
The removal of hydrogen sulfide from waste gases liberated in the course of various chemical and industrial processes, such as wood pulping, natural gas and crude oil production and petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide containing gases not only have an offensive odor, but such gases can cause damage to vegetation, painted surfaces and wildlife, and can also constitute a significant health hazard to humans. Regulations by federal and state governments have imposed increasingly lower tolerances on the amount of hydrogen sulfide that can be vented to the atmosphere. Many localities now require the removal of virtually all the hydrogen sulfide under the penalty of a ban on continuing operation of a plant or facility which produces the hydrogen sulfide-containing gaseous stream.
Alkanolamine process units remove hydrogen sulfide (H.sub.2 S) and carbon dioxide (CO.sub.2) from gaseous process streams, typically by countercurrently contacting an aqueous solution containing from about 20% to about 50% by weight of an alkanolamine with a gas stream containing H.sub.2 S and/or CO.sub.2. For the present application, it is understood that the terms "alkanolamine," "amine" and "ethanolamine" are generic terms including, but not limited to, monoethanolamine ("MEA"), diethanolamine ("DEA"), triethanolamine ("TEA"), diglycoalamine ("DGA") and methyl diethanolamine ("MDEA"). Solutions of water and one or more of the alkanolamines are widely used in industry to remove hydrogen sulfide and carbon dioxide from such gaseous streams. When gases containing hydrogen sulfide and/or carbon dioxide are contacted by a solution of an aqueous amine, the hydrogen sulfide and/or carbon dioxide dissolve in the solution to form weak acids.
H.sub.2 S and CO.sub.2 are not the only gases found in gas emissions which form weak acids when dissolved in water. Other such acid gases, as they are commonly called, that frequently are present in gas streams treated with alkanolamine include sulfur dioxide (SO.sub.2), carbonyl sulfide (COS) and hydrocyanic acid (HCN). When contacted with a solution of an aqueous amine, these gases undergo reactions similar to H.sub.2 S and C.sub.2 and form alkanolamine salts. These salts, however, cannot be removed by conventional steam stripping methods that are often used to remove H.sub.2 S and CO.sub.2 salts and, consequently, they remain and accumulate in the system.
Another problem that is often found in alkanolamine systems occurs when oxygen gets into the alkanolamine system. Oxidation of acid gas conjugate base anions leads to the formation of other alkanolamine salts, most commonly salts of thiosulfate S.sub.2 O.sub.3.sup.= and sulfate SO.sub.4.sup.=. Alkanolamine salts are also formed with thiocyanate (SCN.sup.)) and chloride (Cl.sup.)). Alkanolamine streams containing these salts also cannot be regenerated by conventional steam stripping methods. The oxidation also results in the formation of formates and acetates.
Alkanolamine salts which cannot be heat regenerated, called heat-stable salts, reduce the effectiveness of alkanolamine treating systems. The alkanolamine is protonated and cannot react with H.sub.2 S and CO.sub.2, which dissolve into the solution. Also, accumulated alkanolamine salts can cause corrosion in carbon steel equipment which is commonly used in amine systems. These salts are also known to cause foaming problems which further decreases treating capacity.
Corrosion in alkanolamine units significantly increases both operating and maintenance costs. The mechanisms of corrosive attack include general corrosive thinning, corrosion-erosion, and stress-corrosion cracking. Corrosion control techniques include the use of more expensive corrosion and erosion resistant alloys in the piping and vessels, continuous or periodic removal of corrosion-promoting agents in suspended solids by filtration, activated carbon adsorption, and the addition of corrosion inhibitors. (See Kohl, A. L. and Reisenfeld, F. C., Gas Purification, Gulf Publishing Company, Houston, 1979, pp. 91-105, as well as K. F. Butwell, D. J. Kubec and P. W. Sigmund, "Alkanolamine Treating," Hydrocarbon Processing, March 1982.)
The acid gas sorption capacity in a circulating alkanolamine-water system decreases with time on stream in the absence of added makeup alkanolamine and the system becomes less efficient. This performance degradation is partially attributable to the accumulation of heat stable salts in the alkanolamine-water stream. U.S. Pat. No. 4,795,565 to Yan describes a process for removing heat stable salts from an ethanolamine system by the use of ion exchange resins. The disclosure of U.S. Pat. No. 4,795,565 to Yan is incorporated herein by reference for the operating details both of an ethanolamine acid gas sorption system as well as for the heat stable salt removal process. Yan teaches that strongly acidic and basic cationic and anionic exchange resins are preferred to remove accumulated salts from ethanolamine solutions. Yan also teaches the regeneration of the ion exchange resins using a solution of (NH.sub.4).sub.2 CO.sub.3, NH.sub.4 HCO.sub.3, NH.sub.4 OH or a mixture thereof.
Various processes have been proposed for the regeneration of anion exchange resins used for the reclamation of alkanolamine solutions. U.S. Pat. No. 2,797,188 to Taylor et al. discloses methods for regenerating an alkanolamine absorbent resin bed using sodium hydroxide, either alone or in combination with sodium sulfate. In U.S. Pat. No. 5,162,084 to Cummings et al., an alkanolamine absorbent resin bed is regenerated with sulfuric acid and an alkali metal hydroxide. U.S. Pat. No. 4,970,344 to Keller, U.S. Pat. No. 5,006,258 to Veatch et al. and U.S. Pat. No. 5,788,864 to Coberly et al. disclose methods for regenerating an alkanolamine absorbent resin bed that include a water flushing step and the introduction of sodium hydroxide to remove the thiocyanate ions.
Heat stable salts may also be removed from an alkanolamine system by distillation. However, such separation has been limited in the past to relatively mild conditions of temperature and pressure to avoid thermal degradation of the alkanolamine. For example, diethanolamine ("DEA") boils at 268EC at 760 mm Hg pressure and tends to oxidize and decompose at high temperature. For this reason, vacuum distillation has not been widely used to separate heat stable salts from spent alkanolamine solutions.
The chemistry of alkanolamine degradation is discussed in the Butwell et al. article cited above. The Butwell et al. article notes that monoethanolamine ("MEA") irreversibly degrades to N) (2) hydroxyethyl) ethylene diamine ("HEED"), which has reduced acid gas removal properties and becomes corrosive at concentrations of at least about 0.4% by weight.
Diglycolamine ("DGA"), on the other hand, produces a degradation product upon reaction with CO.sub.2 which exhibits different properties. DGA, a registered trademark of Texaco, Inc., identifies an amine having the chemical formula NH.sub.2)C.sub.2 H.sub.4)O)C.sub.2 H.sub.4)OH. DGA degrades in the presence of CO.sub.2 to form N,N') bis(hydroxyethoxyethyl) urea ("BHEEU") which is similar to HEED in corrosivity but differs in that BHEEU has no acid gas removal properties.
DEA reacts with CO.sub.2 to form N,N'- di(2-hydroxyethyl) piperazine. Unlike HEED and BHEEU, the piperazine compound is noncorrosive and has acid gas removal properties essentially equal to its parent, DEA. See the Butwell et al. article at page 113.
Diisopropylamine ("DIPA") readily degrades when contacted with CO.sub.2 to form 3-(2-hydroxypropyl) 5-methyl oxazolidone which has essentially no acid gas removal properties. See the Butwell et al. article at page 113.
Numerous degradation products formed by the reaction of H.sub.2 S, or a mixture of H.sub.2 S and CO.sub.2 with diethanolamine have been reported from analyses of operating diethanolamine acid gas sorption processes. The complex chemistry of alkanolamine degradation may account at least in part for the unpredictable behavior of ion exchange resins for removing heat stable salts from aqueous alkanolamine solutions.
The regeneration of the ion exchange resin beds is the most difficult task for an alkanolamine reclamation process. There have been several attempts made to use ion exchange technology for reclamation in amine systems, but most have been unsuccessful due to an unacceptable regeneration process. Water treatment plants have successfully used ion exchange technology, but the total anion concentrations being removed is significantly lower and the affinity of the anions being removed is generally much less, making it easier to regenerate the ion exchange resin beds. In contrast, an amine system is exposed to very high levels of anions (as high as 45,000 ppmw) and more repetitive regeneration cycles are required to remove these anions. Also, stronger anions, such as thiocyanate and sulfate, require a different regeneration method due to the difficulty of removing these anions from the cationic sites of the strong base resin.
The methods presently being used to regenerate ion exchange resin beds in alkanolamine reclamation systems commonly requires a two stage regeneration method with the first stage utilizing a strong acid such as sulfuric acid or hydrochloric acid. These acids are difficult to handle and pose health and safety hazards to personnel and maintenance problems to the equipment due to their corrosivity. Consequently, there is a need for a process for the efficient regeneration of ion exchange beds that does not use corrosive acids.